Vehicle transmissions include various gear positions or settings, commonly denoted as PRNDL for park, reverse, neutral, drive, and low drive, respectively. The PRNDL settings are selectable by a vehicle operator by moving a shift lever or other shift mechanism to thereby command or initiate a shift request. When shifting into reverse, for example, conventional or mechanically geared vehicle transmissions generally require moving or shifting a gear arrangement into a particular configuration suitable only for the reverse operation, i.e. vehicle propulsion in the rearward direction. With such conventional transmissions, the engine, which rotates or spins in only one direction, must provide rotational force or torque to propel the vehicle in both forward and reverse operating directions. Thus, reverse gearing must be configured to alter the direction of engine rotation at the output of the transmission.
Therefore, with conventional transmissions the reverse shift is prevented under certain circumstances. For example, if the vehicle is moving forward at a particular speed in excess of a predetermined threshold speed, reverse shifts may be disabled or prevented so as to protect the engine and/or transmission from excessive shift shock. Likewise, when the vehicle is traveling at a relatively high rate of speed in a reverse direction, a “forward/drive” shift request may be disabled or prevented. Such shift control functionality may be provided using integrated transmission controls or other methods and/or control devices suitable for preventing the shifting event.
In contrast to vehicles having a conventional transmission, hybrid vehicles are alternately or concurrently powered by an engine and/or one or more electric motor/generators, with forward and reverse operations typically achieved without specific gearing changes or shifting. In such hybrid transmissions, forward and reverse operations are achieved with the same mechanical configuration within the transmission. For example, “forward/drive” is achieved by commanding transmission output torque in a positive direction, while “reverse” is achieved by commanding transmission output torque in a negative direction. Therefore, unlike many conventional transmissions, hybrid transmissions may execute a reverse shift under limited forward operating conditions, or a forward shift under limited reverse operating conditions. However, both conventional and hybrid transmission control methodologies may be less than optimal when operating under certain drive conditions, particularly when the vehicle is traveling at a relatively high rate of speed.